When serially connected batteries are repeatedly charged and recharged, the batteries end up with different voltage levels due to different life of the batteries. When the voltages having different voltage levels are repeatedly charged and discharged, the batteries that are more deteriorated and nearer to the useful life will tend over-charge and over-discharge during charge/discharge operations and it will accelerate the deterioration process and lower the overall performance of the battery pack. For efficient use of a battery pack, it is important to uniformly charge and discharge the serially connected batteries, and for this, the voltage levels of the batteries are kept at a same level. This is called cell-balancing, and the present invention is proposed to satisfy this need.
FIG. 1 shows the battery charging concept in accordance with general battery management system.
As shown in FIG. 1, a plurality of batteries BAT1-BAT4 are connected in series.
Between the cathode and anode terminals of each battery BAT1, BAT2, BAT3, BAT4, a current path forming part is provided. The current path forming part includes a resistor R and a switch SW, and it provides a current path when the associated battery is fully charged so as to prevent the overcharging.
Charging electricity providing unit is connected to anode of the first battery BAT1 and the cathode of the fourth battery BAT4 among the plurality of batteries BAT1-BAT4 and provides the charging electricity.
If the batteries BAT1, BAT3, BAT4 are fully charged, the switches SW1, SW3, SW4 of the current path forming parts for the batteries BAT1, BAT3, BAT4 are turned on such that the current is redirected to the associated resistors R1, R3, R4. That is, the current is bypassed to resistance for each of the fully charged batteries BAT1, BAT3, BAT4.
This type of conventional battery management system allows continuing current flow into resistance even when a battery is fully charged, and this is outputted in form of heat increasing the electricity consumed and lead to wasted energy. The energy waste gets worse as more number of fully charged batteries are present in the battery pack, and as more number of bypassed current path to resistance.
Even if the current is bypassed into the current path leading to resistance, voltage may still be applied to the fully charged battery due to leakage current to cause overcharging of the battery, and this can lead to shortened life of the battery.